Hydroperoxides are used in many ways in the chemical industry and engineering, e.g. as catalysts or curing agents in polymerization reactions, as sources of oxygen in blowing agents and as intermediates for the preparation of the corresponding alcohols of peroxides and peracid esters. In this respect, tert.aralkyl hydroperoxides in particular are of great importance.
Tert.aralkyl hydroperoxides are generally prepared by oxidation of the corresponding hydrocarbons with molecular oxygen, conversion rates of only 30 to 35% being obtained; at higher conversion rates, the selectivity falls considerably, i.e. by-products are formed to an increasing degree. The hydroperoxides must then be concentrated by distillation, crystallization or extraction, or separated from unreacted hydrocarbon starting product.
Two methods for the preparation of tert.hydroperoxides by reaction of tert.alcohols with hydrogen peroxide with acid catalysis are known from the literature, which processes operate with different concentrations and quantities of sulphuric acid: R. Criegee and H. Dietrich (Annalen 560 (1948) 135) operate using 80 to 95% H.sub.2 O.sub.2 and in the presence of low concentration sulphuric acid; and Milas and Harries (J.Am.Chem Soc. 60 (1938) 2434) operate using 30% H.sub.2 O.sub.2 in the presence of a large quantity of 70% sulphuric acid. The process according to R. Criegee and H. Dietrich is described by the authors themselves as unsuitable for relatively large batches, and it is admitted that the older process according to Milas and Harries is cheaper and safer and is, therefore, the only one suitable for relatively large batches. The procedure according to Milas and Harries does, however, have disadvantages in the case of the preparation of tert.aralkyl hydroperoxides; these hydroperoxides are very sensitive to strong acids and decompose into ketones and phenols.
According to M. S. Belenkiy et al (The Soviet Chem.Ind. 1 (1972) 16), aryl-alkyl-carbinols contained in mixtures obtained from the oxidation of alkyl aromatics with air are oxidized with hydrogen peroxide to the hydroperoxides. High conversion rates and, hence, good yields can be achieved only if high temperatures and/or relatively high acid concentrations are used. The optimum conditions stated represent a compromise between the formation and decomposition of the hydroperoxides.
A. Burghardt et al (Chemia Stosowana, Ser. A, Volume 13, No. 4 (1969) 335-342) describe the reaction of methyl ethyl phenyl carbinol with hydrogen peroxide in the presence of sulphuric acid and a solvent to sec.-butylbenzene hydroperoxide; due to the large quantity of non-polar solvent (four times the amount of carbinol), the hydroperoxide formed is protected from decomposition by acid.
According to A. Burghardt et al (Zess.Nauk.Politech.Slask., Chem. No. 60 (1972) 3-20), cumene hydroperoxide is prepared from dimethylphenyl carbinol and hydrogen peroxide in the presence of a solvent. Sulphuric acid and/or acid cation exchangers are used as acid catalyst. With this process, too, operations are not carried out without solvent and no pure hydroperoxide is produced. If a solid hydroperoxide were prepared, separation of the ion exchange resin would also be extremely difficult.
Even in the applicant's own tests for the preparation of the hydroperoxides in the pure form from pure carbinols with H.sub.2 O.sub.2 /mineral acid, there was an insufficient reaction and, hence, a low yield and purity of the product, or a tendency towards decomposition which leads to considerable discoloration of the product. The acid catalyzed decomposition of tert. aralkyl hydroperoxides is, moreover, highly exothermic and therefore also represents a high safety risk.